Harmonics of the AC susceptibility for the study of I-V curves in melt grown YBCO

The measurements of the first and the third harmonics of the AC magnetic susceptibility, performed on melt grown YBCO samples as a function of the temperature and the external parameters (frequency and amplitude of AC magnetic field, intensity of a DC field) were studied by using the Cole-Cole plot analysis. By considering the AC field dependence of the third harmonics Cole-Cole plots, we have extracted information about the peculiar behaviour of the I-V characteristics and, therefore, about the dissipative phenomena in the sample. The experimental results have been analysed with the help of both analytical and numerical curves. In particular, the use of a phenomenological expression for the resistivity allowed us to reproduce the main experimental features, including the observed invariance of the area inside the Cole-Cole plots and their behaviour as a function of the AC field. (C) 2003 Elsevier B.V. All rights reserved.